Basketball players dominate the list (40%) followed by American football and football (soccer) players who make up almost 30% of the top earners.
Forbes said Messi's annual salary and bonus was more than £59m before tax, making him the year's highest-paid player, with endorsement deals with companies including Adidas and Pepsi adding to his fortune.
With an additional $13 million in endorsements, Mayweather earned a total of $375 million between June 1 2017 and June 1 2018, comfortably eclipsing the second-placed figure on the list, football star Lionel Messi, who made $146 million.
Williams was the only woman to feature in the top 100 last year, but her prize money dropped from $8 million to $62,000 this year after she gave birth to her daughter, Alexis Olympia Ohanian Jr., in September. That stems largely from his bout last August with UFC's Conor McGregor, who is at No.4 on this year's ranking ($99 million).
Unsurprisingly, the National Basketball Association dominates the list, with 40 players, 19 of whom are in the top 50 as opposed to the NFL's nine.
NBA's top earner LeBron James banked Dollars 85.5 million, including endorsements, over the last 12 months to rank sixth among the highest-paid athletes.
Brazilian footballer Neymar rounded out the top five.
There are no female athletes on the list.
Between them, the top 100 earned a collective $3.8bn during the past 12 months, which represents an increase of 23% from 2017.
Golden State Warriors star Stephen Curry was the only other National Basketball Association player to feature in the top 10, with earnings of $101 million.
The top is dominated by American sportsmen including LeBron James and two National Football League quarterbacks, but there is no room for New England Patriots' hero Tom Brady.
Forbes said it was harder than ever to qualify for the 100 highest-paid athletes, with the cutoff up Dollars 1.5 million to USD 22.9 million.